Power supplies, or ballasts, are typically used in powering gas discharge lighting to convert the low-frequency, low impedance, low voltage power source available from a 120 Volt 60 Hz AC wall outlet into a high frequency, high voltage and high impedance source suitable for connection to the gas discharge tube light. Typically a ballast includes a rectifier for rectifying the 120 Volt AC source into an approximately 170 Volt DC source, an oscillator for converting the DC source into a high frequency AC source, and a transformer for stepping up the voltage of this high frequency AC source.
Because the output impedance of a ballast is typically relatively large, the output voltage of the ballast varies widely depending on the load. Many known ballasts suffer from excessive output voltage when operated without load or with a predominantly capacitive load. This may be due to the resonance between the effective transformer output impedance and the output (or stray) capacitance within the transformer or connected to its secondary. Excessive output voltage produces excessive stress on the internal insulation of the transformer, the insulation of the high voltage wiring leading from the ballast, and the insulating materials at the support for holding the gas discharge tube light. The excessive output voltage also violates certain agency safety requirements.
Recently, dimmable light ballasts have been introduced, and are increasingly in demand. A dimmable ballast has a bright mode, for daytime use, and a dim mode for nighttime use. Dimming may be achieved by reducing the current supplied to the light tubing. One known approach to reducing the tubing current is regularly stalling the operation of the oscillator in the ballast, thus providing fewer oscillator cycles to the transformer in dim mode than are provided in the bright mode. However, this approach requires relatively complex control circuitry; furthermore, the frequency at which the oscillator is stalled may be in the audible range, causing audible mechanical vibration of the ballast which may be unacceptably noisy.